1. Field of the Invention
The invention relates to a control system for controlling a running direction of an automotive vehicle, and, in particular, to a vehicle stability control system for controlling a braking system to apply braking force independently to wheels of the vehicle so as to direct the vehicle toward a target direction.
2. Description of the Related Art
There have been known various vehicle attitude control systems which are typically comprised of a braking system capable of applying braking force to each of four wheels and a control unit to control the braking system to apply braking force to the wheels independently so as to rectify a running attitude of the vehicle. Such an vehicle stability control system which is known from, for instance, Japanese Unexamined Patent Publication No. 7-232629, monitors variables representing a running attitude of a vehicle with respect to a running direction and controls the braking system according to the running attitude determined on the basis of the variables and causes the braking system to apply braking force to the wheels selectively and independently so as to impart an appropriate yaw moment to the vehicle. The vehicle stability control system relieves the tendency toward understeering or oversteering by means of applying a yaw moment to the vehicle and, as a result of which, prevents the vehicle from drifting out and/or spinning. The prior art vehicle stability control system adapted to change braking force applied to each wheel on the basis of an attitude has to apply strong braking force to each wheel in the event where the vehicle turns a corner with a significant deviation in attitude with respect to a target running direction. If the braking force is too strong for the wheels to provide their greatest tire gripping force, some of the wheels will be locked up. The tire gripping force is the resultant force of a longitudinal force acting in the running direction and a lateral force produced due to inertia in a direction perpendicular to the running direction. If the braking force becomes as large as the maximum longitudinal gripping force, the wheel tires can not produce sufficient lateral gripping force, making the vehicle impossible to turn sharply on a corner. In particular, if the vehicle suffers a strong tendency toward understeering, the front wheels are apt to be locked up, which leads to an unstable running attitude of the vehicle and forces, in certain cases, the vehicle to drift out of the cornering path.